The integration of geomechanics and wellpath design is currently a subject of various research efforts. Generally, the proposals published to date are modified workflows which incorporate stability analysis within the overall process of determining well trajectory for a given situation. Such modified workflows attempt to reconcile the different, and sometimes contradictory, goals of achieving borehole stability and reaching one or more positions at different depths in a formation. Currently, the state of the art is a workflow that combines the two problems by executing a pre-processing step and a post-processing step.
The pre-processing step includes calculation of a subset of geometric conditions which satisfy user-defined stability criteria. The results are translated into corresponding wellbore positions, and may be presented to the engineer as colored polar plots indicative of stress distribution around a borehole for various combinations of inclination and azimuth. By iteratively modifying inclination and azimuth for sets of controllable and uncontrollable variables it is possible to produce an instability indicator based on selected failure criterion. This allows calculation of maximum and minimum values of any other variable to achieve stability, e.g., minimum rock strength to prevent shear failure. However, there is no nexus between positions, and wellpath selection is a function of individual manipulation and interpretation by the engineer.
The post-processing step is employed after the wellpath is selected by the engineer. In particular, based on an n-dimensional geomechanical model, post-processing generates a depth profile of drilling fluid density to prevent shear and tensile failures of the borehole walls. This data is employed to calculate a requisite drilling mud weight. Note that this does not improve the wellpath solution provided by the pre-processing step, but rather helps to compensate for deviation from an optimal wellpath solution by calculating drilling mud weight requirements to prevent failure of the least geomechanically stable positions.
One of the drawbacks of the two-step workflow described above is that solutions are heuristic and determined manually. For an n-layer geomechanical model, where for each layer a polar plot will be computed, a set of n suggested wellbore positions is associated with a corresponding depth. Given a wellhead at a starting point P1 and a target position P2, the engineer attempts to manually find a path P1-P2 which satisfies the set of pre-processed position suggestions at each depth. This process is relatively slow because it is manual. Further, the process is heuristic because the relative strengths of different potential trajectories may not be apparent to the engineer without some analysis, i.e., the engineer cannot pick the best trajectory out of the data, but rather picks various potential trajectories for comparison. As a result, the selected wellpath may be far from optimal.